INTRODUCTION:
Simultaneous production of chlorine, Caustic soda solution and hydrogen from aqueous solution of alkaline chlorides by application of direct current, commonly known as the electrolysis of alkaline chlorides has been widely adopted in the chemical industry during the past 70 years. A rapid development created a great variety of processes and cell types but only a few well engineered designs have been retained for present commercial application.
BASICALLY THREE DIFFERENT METHODS ARE PRESENTLY EMPLOYED:-
The electrolysis using graphite anodes or metal anodes and mercury cathodes -the amalgam process.
The electrolysis using graphite and iron cathodes partitioned by diaphragms -the diaphragm proces.
The electrolysis using metal anodes and cathodes partitioned by cation exchange membranes – the membrane process.
While the mercury cell and the diaphragm cell process for the electrolysis of alkaline chlorides and still part of a traditional field activity, many companies has been engaged in research and the development of the membrane cell process since the early seventies. The membrane process had indeed, passed through various stages or design and development. It has become the consensus of expert opinion that the membrane electrolysis will be the predominant process for chlor alkali production in the future. This is based on the following advantages.
1. Reduced energy consumption in the membrane chloralkali process through the utilization of the perfluoro – carbonic membrane suitable for production of 30 – 35% NaOH.
2. Lower investment cost due to simplicity of electrolysis cell and less space requirement for electrolysers.
3. Ease of operation high operational flexibility.
4. Lower operation cost due to high life of the membrane and also due to less personnel requirement for cell operation and maintenance.
5. High product purity (less that 100 PPM NaCl in 32% NaOH and practically no hydrogen in chlorine gas).
6. No environmental pollution due to mercury or asbestos or any other substances.
PRINCIPLES OF THE MEMBRANE PROCESS :
The Ion exchange membrane itself is the heart of the membrane cell system. It acts as partition between the anode and the cathode compartment of the cell. The effectiveness of the membrane as separation device between the anode and the cathode compartments defines the current efficiency of the cell. The chemical composition of the membrane, based on the fluorocarbon matrices, defines the range of caustic concentration at which an optimum operating performance is given.
Membranes are currently available to produce caustic concentrations of up to 35% NaOH with optimum performance. The physical and electrochemical properties and Ion exchange capability can vary widely. A minimum of 3 years life expectancies are routinely quoted from an economical standpoint.
The Ion exchange membrane is impermeable for liquid and gas. It is selectively permeable to Na+ cation and the passage of OH-
The membrane effectively allows, the passage of Na+ ions only and prevents the diffusion of anions from the anode compartment to the cathode compartment. Thereby making it possible to obtain caustic soda of a very high purity.
CELL DESIGN AND FUNCTION :
The 81 elements of one electrolyser are suspended in a steel frame. For current transmission the individual elements of a electrolyser are divided into 4 groups of 20 (21) elements each and pressed against the fixed contact points of the frame by means of a preloaded clamping device. The piping is located underneath the cells.
The cell elements of the so-called single element design, the half shells being welded to the electrodes. Titanium is used for the anodes and nickel for the cathodes.
The ion exchange membrane is clamped between the half shells with interposed gaskets. The half shells are bolted together at the flange, thus constituting a cell element. The current is conducted from cell to cell by means of contact strips.
The brine and the catholyte are admitted at the bottom of the cell. While anolyte and catholyte are likewise discharged downwards via long internal stand pipes, where the electrolytes overflow to maintain the level in the cell.
The feed piping is connected with the elements by means of long, thin plastic hoses. This arrangement and other design measures prevent current leaks due to potential differences between the cells, which would have a detrimental effect.
Each element is sealed at the flange by belting. The bolts are insulated to prevent current transmission are located outside the electrolytes and only minor compressive forces are necessary to transmit the current to the neighboring elements. These designs ensured that the housing and electrode are at the same potential, whereby corrosion within the cell is prevented.
ADVANTAGES OF THIS DESIGN MAY BE SUMMARIZED AS FOLLOWS:-
1) The replacement of one single element required draining of one element group, disengagement of the contacts by loosening of the thrust elements and disconnecting the hose connection between the cell element and the supply and discharge lines.
2) All other elements within a cell stack can remain in-situ.
3) The removed element is replaced by a new element. Thus the time required for replacing one or several elements is relatively short.
4) The cell elements are lifted out of the cell stack by means of the cell crane and transported to the cell workshop area.
5) As the elements are sealed by bolting, only minor compressive forces are required for the electrical contacts.
6) Monitoring of electrolyte flow outside of each element.The each cell connected in parallel, so its called bipolar. The cell contains 81 elements. Each element contains an anode, cathode, and a membrane. The Nafion 966 membrane is used. The anode is made up of titanium metal. The cathode is made up of Nickel.
MEMBRANE COATING:
Anode side coating is sulfonic acid.
Cathode side coating is carboxylic acid.
CELL REACTION:
The gross reaction for the formation of chloride, caustic soda and hydrogen form a sodium chloride solution can expressed as follows.
NaCl + H2O ® NaOH + ½ Cl2 + ½ H2
This reaction is taking part as two separated cell electrode reactions, the anode and cathode reaction.
ANODE REACTION:
Cl- ® ½ Cl2 + e-
CATHODE REACTION:
H2O ® H + + OH-
H2O + e- + Na + ® NaOH + ½ H2
SIDE REACTIONS AND INEFFICIENCIES:
The main reaction at the anode is electrolytic oxidation of chloride ions to chlorine.
2Cl- ® Cl2 + e-
The chlorine evolved is the desired product, however some chlorine is partially dissolved in the water and reacts accordingly.
Cl2 + H2O ® HOCl + H+ + Cl - -----(1)
The hypochlorous acid formed in reaction -1 is a weak acid that readily dissociates.
HOCl ® OCl- + H+ ------ (2)
The hypochlorous acid and the hypochlorite ion (OCl-) originating from 1 and 2 can give rise a third reaction, which is also purely chemical in nature and produces chlorates.
2HOCl + OCl- ® ClO3 + 2H+ + 2Cl- ------ (3)
By combining reaction 1, 2 and 3 it is possible to write down the following equation.
3Cl2 + 3H2O ® ClO3- + 6H+ + 5Cl - ------ (4)
The formation rate of chlorate, which reduced the cell efficiency, is clearly a function of several parameters.
1. The partial pressure of chlorine.
2. The concentration of Cl- ions, i.e. the concentration of salt in anolyte.
3. The pH of the anolyte. The amount of chlorate formed can minimize by increasing the concentration of Cl- ions and reducing the anolyte pH.
4. Chlorate may also formed in lesser extent electrochemical at the anode, i.e. according to a primary reaction involving hypochlorite ions.
6OCl- + 3H2O - ® 2ClO3- + 6H+ + 4Cl- + 1.5O2 + 6e- ----(5)
This reaction is favored by decreasing the acidity of the anolyte.
An other important side reaction on the anode, which is responsible for a further loss of current efficiency, is the generation of oxygen at the anode.
2H2O ® O2 + 4H+ + 4e- ---- (6)
Deposits the standard electrode potential for this reaction is 1.22 a V at 250C, i.e. less than E0 Cl2 / Cl- = 1.358, only minor amounts of O2 are formed. This is due to the high oxygen over potential at the anode coating by which the discharge potential of O2 is higher than that of the Cl2.
The rate of O2 generation depends on the available concentration of OH- OH-
